Currently, information is displayed on sheets using permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information. Magnetically written data, however, is not visible.
World patent application PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages”, is a thorough recitation of the art of thin, electronically written sheet display technologies. Disclosed is the assembling of multiple display sheets that are bound into a “book”, each sheet provided with means to individually address each page. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bistable liquid crystal system, and thin metallic conductor lines on each page.
An early patent, U.S. Pat. No. 3,578,844, discloses a light modulating structure for a display device. In the patent, cholesteric liquid crystal material is encapsulated by light penetrable gelatin and gum arabic capsules that are coated on a screen. The capsules were formed by emulsifying the cholesteric material in a gelatin solution using a blender to form droplets between 10 and 30 microns in diameter. The pH of the emulsion was changed to precipitate a gelatin coating over each droplet of cholesteric material. The gelatin was hardened and the capsules sieved from the solution. The capsules are then coated over a field-carrying surface that is in contact with heating elements to provide an electrically switchable image that must be heated to isotropic to erasure written images.
U.S. Pat. No. 3,600,060 to Churchill et al. discloses another process for providing cholesteric liquid crystals in a polymer matrix. The patent discloses emulsifying droplets of liquid crystal in a solution having a dissolved film forming polymer. The patent further discloses coatings or films having droplets of cholesteric liquid crystal material between 1 and 50 microns in diameter. Suitable binders mentioned in the paper include gelatin, gum arabic, and other water soluble polymers. Churchill et al. disclose the emulsion can be coated on a substrate, for example, by means of a draw down applicator to a wet thickness of about 10 mils and air dried at about 25° C. Churchill et al. state that the layers can be dried to touch. This technique of emulsifying droplets of liquid crystal in a solution allows for droplets with a large variation in diameter and will not allow for multiple well controlled droplet sizes. This creates large domains and smaller parasitic domains. Parasitic domains have reduced optical characteristics compared to optimized larger domains.
Another technique for providing liquid crystal domains in a coating is disclosed as prior art in U.S. Pat. No. 4,673,255. A miscible mixture of liquid crystal and polymer are combined. Phase separation is accomplished by curing or polymerizing a solution of resin and liquid crystal; by cooling a solution of liquid crystal and thermoplastic resin heated to a temperature sufficient to dissolve the liquid crystal; or by evaporating a solvent in which the polymer and liquid crystal are mutually soluble. The resin polymer is phase separated from the liquid crystal to form microdroplets of the liquid crystal in a polymeric matrix. The step of controlling the rate of microdroplet growth can be carried out by controlling the rate of cooling of a homogeneous solution of liquid crystal and thermoplastic polymer heated to a temperature sufficient to dissolve the liquid crystal. The phase separation can be thermally induced, solvent induced or polymerization induced to create domains of liquid crystal. This method allows for microdroplet growth so that the microdroplets have a selected average diameter when phase separation is complete. This method cannot be implemented to improve packing density and the overall brightness of the display since it will not allow for multiple precisely controlled droplet sizes. This method is predominantely used with nematic liquid crystals.
U.S. Pat. No. 6,423,368 to Stephenson et al. discloses dispersing a liquid crystal material in an aqueous bath containing a water soluble binder material such as gelatin, along with a quantity of colloidal particles wherein the colloidal particles limit coalescence. The limited coalescent materials were coated over a substrate and dried, wherein the coated material formed a set of uniform limited coalescence domains having a plurality of electrically responsive optical states. The limited coalescence method allows the production of material with uniform sized domains or droplets of liquid crystalline material. However, the final coating has droplets of approximately all the same size, which can limit packing density, resulting in non-optimized reflectance and an increase in surface roughness.
U.S. Pat. No. 6,061,107 discloses that controlling the shape of domains of liquid crystal material in a polymer binder can improve light scattering properties. The patent discloses the use of temperature, solvent and polymer induced phase separation techniques to provide domains of liquid crystal that have a major and minor axis, where the major axis is larger in size than the cell thickness. When solvents are used, there may be significant environmental concerns. U.S. Pat. No. 6,061,107 also discloses liquid crystal material having monomers in solution that are polymerized to form polymer dispersed liquid crystals. This method does not allow for multiple well controlled droplet sizes in one simultaneous coating.
U.S. Pat. No. 6,704,073 to Stephenson et al. discloses the coating of polymer-dispersed liquid crystals comprising the steps of providing an emulsion having cholesteric liquid crystal material in a gelatin solution, heating the emulsion to reduce the viscosity of the emulsion, coating the heated emulsion on a substrate, lowering the temperature of the coated emulsion to change the state of the coated emulsion from a liquid to a gel state, thereby forming a coating characterized by a corresponding increased viscosity state, and drying the coating, while maintaining it in the high viscosity state, to form a coating in which domains of polymer dispersed cholesteric liquid crystals are dispersed in a gelatin-containing matrix. However, the final coating has droplets of approximately all the same size, which can limit packing density, resulting in non-optimized reflectance and an increase in surface roughness.